This invention provides an improved method and apparatus for determining the trajectory of boreholes to directional and horizontal targets. In particular, the improved technique replaces the use of a preplanned drilling profile with a new optimum profile that maybe adjusted after each survey such that the borehole from the surface to the targets has reduced tortuosity compared with the borehole that is forced to follow the preplanned profile. The present invention also provides an efficient method of operating a rotary steerable directional tool using improved error control and minimizing increases in torque that must be applied at the surface for the drilling assembly to reach the target.
Controlling the path of a directionally drilled borehole with a tool that permits continuous rotation of the drillstring is well established. In directional drilling, planned borehole characteristics may comprise a straight vertical section, a curved section, and a straight non-vertical section to reach a target. The vertical drilling section does not raise significant problems of directional control that require adjustments to a path of the downhole assembly. However, once the drilling assembly deviates from the vertical segment, directional control becomes extremely important.
FIG. 1 illustrates a preplanned trajectory between a kick-off point KP to a target T using a broken line A. The kickoff point KP may correspond to the end of a straight vertical segment or a point of entry from the surface for drilling the hole. In the former case, this kick-off point corresponds to coordinates where the drill bit is assumed to be during drilling. The assumed kick-off point and actual drill bit location may differ during drilling. Similarly, during drilling, the actual borehole path B will often deviate from the planned trajectory A. Obviously, if the path B is not adequately corrected, the borehole will miss its intended target. At point D, a comparison is made between the preplanned condition of corresponding to planned point on curve A and the actual position. Conventionally, when such a deviation is observed between the actual and planned path, the directional driller redirects the assembly back to the original planned path A for the well. Thus, the conventional directional drilling adjustment requires two deflections. One deflection directs the path towards the original planned path A. However, if this deflection is not corrected again, the path will continue in a direction away from the target. Therefore, a second deflection realigns the path with the original planned path A.
There are several known tools designed to improve directional drilling. For example, BAKER INTEQ""S xe2x80x9cAuto Trakxe2x80x9d rotary steerable system uses a closed loop control to keep the angle and azimuth of a drill bit oriented as closely as possible to preplanned values. The closed loop control system is intended to porpoise the hole path in small increments above and below the intended path. Similarly, Camco has developed a rotary steerable system that controls a trajectory by providing a lateral force on the rotatable assembly. However, these tools typically are not used until the wellbore has reached a long straight run, because the tools do not adequately control curvature rates.
An example of controlled directional drilling is described by Patton (U.S. Pat. No. 5,419,405). Patton suggests that the original planned trajectory be loaded into a computer which is part of the downhole assembly. This loading of the trajectory is provided while the tool is at the surface, and the computer is subsequently lowered into the borehole. Patton attempted to reduce the amount of tortuosity in a path by maintaining the drilling assembly on the preplanned profile as much as possible. However, the incremental adjustments to maintain alignment with the preplanned path also introduce a number of kinks into the borehole.
As the number of deflections in a borehole increases, the amount of torque that must be applied at the surface to continue drilling also increases. If too many corrective turns must be made, it is possible that the torque requirements will exceed the specifications of the drilling equipment at the surface. The number of turns also decreases the amount of control of the directional drilling.
In addition to Patton ""405, other references have recognized the potential advantage of controlling the trajectory of the tool downhole. (See for example, Patton U.S. Pat. No. 5,341,886, Gray, U.S. Pat. No. 6,109,370, WO93112319, and Wisler, U.S. Pat. No. 5,812,068). It has been well recognized that in order to compute the position of the borehole downhole, one must provide a means for defining the depth of the survey in the downhole computer. A variety of methods have been identified for defining the survey depths downhole. These include:
1. Using counter wheels on the bottom hole assembly, (Patton, U.S. Pat. No. 5,341,886)
2. Placing magnetic markers on the formation and reading them with the bottom hole assembly, (Patton, U.S. Pat. No. 5,341,886)
3. Recording the lengths of drillpipe that will be added to the drillstring in the computer while it is at the surface and then calculating the survey depths from the drillpipe lengths downhole. (Witte, U.S. Pat. No. 5,896,939).
While these downhole systems have reduced the time and communications resources between a surface drilling station and the downhole drilling assembly, no technique is known that adequately addresses minimizing the tortuosity of a drilled hole to a directional or horizontal target.
Applicant""s invention overcomes the above deficiencies by developing a novel method of computing the optimum path from a calculated position of the borehole to a directional or horizontal target. Referring to FIG. 1, at point D, a downhole calculation can be made to recompute a new trajectory C, indicated by the dotted line from the deviated position D to the target T. The new trajectory is independent of the original trajectory in that it does not attempt to retrace the original trajectory path. As is apparent from FIG. 1, the new path C has a reduced number of turns to arrive at the target. Using the adjusted optimum path will provide a shorter less tortuous path for the borehole than can be achieved by readjusting the trajectory back to the original planned path A. Though a downhole calculation for the optimum path C is preferred, to obviate delays and to conserve communications resources, the computation can be done downhole or with normal directional control operations conducted at the surface and transmitted. The transmission can be via a retrievable wire line or through communications with a non-retrievable measure-while-drilling (MWD) apparatus.
By recomputing the optimum path based on the actual position of the borehole after each survey, the invention optimizes the shape of the borehole. Drilling to the target may then proceed in accordance with the optimum path determination.
The invention recognizes that the optimum trajectory for directional and horizontal targets consists of a series of circular arc deflections and straight line segments. A directional target that is defined only by the vertical depth and its north and east coordinates can be reached from any point above it with a circular arc segment followed by a straight line segment. The invention further approximates the circular arc segments by linear elements to reduce the complexity of the optimum path calculation.